Increased insertion number leads to increased sperm transfer and fertilization success in a nursery web spider

Abstract

Across animals, a male's fitness is largely dictated by his ability to fertilize eggs; and there exists a plethora of male adaptations associated with increasing fertilization success. In the nursery web spider, Pisaurina mira, males restrain females prior to and during copulation by wrapping them with silk. Previous research demonstrates that copulatory silk wrapping reduces a male's chance of being sexually cannibalized and increases the number of sperm transfer opportunities (termed insertions) that a male can achieve within a mating. While avoiding cannibalism provides an obvious survival benefit to males, the impact of insertion number on male fitness remains unknown. This study tested the hypothesis that increased insertion number realized through copulatory silk wrapping increases (1) the quantity of sperm transferred and (2) fertilization success. To accomplish this, we directly quantified the amount of sperm in male pedipalps (i.e. the male sperm storage organ) before mating and after obtaining one or two insertions. We also, indirectly quantified fertilization success by measuring the number of hatched offspring when males were capable of achieving one versus two insertions within a mating. In support of our hypotheses, we found that males transfer roughly twice the amount of sperm when achieving two insertions compared to one. We additionally found that the amount of sperm transferred is negatively related to female size. In terms of offspring number, females obtaining two insertions had more offspring compared to females obtaining only one insertion. These results show that males achieve a fertilization benefit from increased insertion number, which is obtained through the male behaviour of copulatory silk wrapping.